The present invention relates to the field of water treatment, and provides a method and apparatus which improves the efficiency of an aeration pond or basin.
Waste water is treated by exposing the water to oxygen. Naturally-occurring organisms in the water need oxygen to enable them to consume the waste. If there is insufficient oxygen in the water, the organisms will tend to come to the surface of the water, to obtain the needed oxygen. This migration of the organisms causes odor in a water treatment facility.
To provide oxygen which is reasonably well dispersed throughout the water in a treatment pond, it has been known to provide oxygen at the bottom of the pond, and allow it to form bubbles which rise through the pond. In the latter system, a source of oxygen is conveyed to a perforated hose, located at or near the bottom of the pond, and the oxygen leaks through perforations located around the perimeter and along the length of the hose. The result is a stream of small bubbles ranging in diameter from about 1-5 mm. As the bubbles of pure oxygen rise in the water, they dissolve in the water, and oxygen is transferred to the organisms in the pond.
The transfer of oxygen in the manner described above is not perfectly efficient. The percentage of oxygen which is transferred depends on the amount of dissolved oxygen in the water, the biological oxygen demand (BOD) of the organisms, the chemical oxygen demand (COD), which is the requirement associated with specific chemicals such as hydrogen sulfide, and on other factors which will be apparent to those skilled in the art. In general, there will always be unreacted oxygen bubbles which reach the surface. The unreacted bubbles may constitute 10-50 percent of the original oxygen supplied. In systems of the prior art, this oxygen has been wasted, as it simply vents to the air above the surface of the pond.
It has been known that the efficiency of oxygen transfer is also directly related to the depth of the water. The deeper the water, the more time for reaction, and the greater the efficiency of oxygen transfer. Still other factors, such as the oxygen saturation point (Cs) for a given water temperature, bubble velocity, bubble surface area, and interfacial boundaries, also contribute to the overall efficiency.
Most aeration basins do not employ pure oxygen to aerate the water. The most common method, in the prior art, for aerating waste water, is to use motor-driven aeration systems to force circulation of the water and thus increase its contact area with the surrounding air. These devices take many different forms, such as downdraft pumps, which force surface water to the bottom, updraft pumps, which produce a small fountain, paddle wheels, and other similar devices. Since air contains only about 21% oxygen, the transfer efficiency of these devices is comparatively low. The initial investment in equipment, and the ongoing expenses of electricity and maintenance for these devices makes their overall cost higher than that of a system which uses pure oxygen. Using pure oxygen to treat waste water also offers the advantage that one can treat a larger volume of water for a basin of a given size.
Another prior art device provides a custom-fitted fabric cover which sits on the surface of an aeration pond. The cover is relatively air-tight, and is intended to prevent obnoxious odors from being discharged. Such a cover is relatively expensive, and installation is difficult. In some applications, oxygen or compressed air can be pumped under the canopy to supply additional oxygen to the process. In the latter case, the transfer of dissolved oxygen occurs only at the surface of the water.
It has also been known to combine the above-described fabric cover with additional devices, such as those described earlier, which circulate, spray, or otherwise increase the effective surface area of the water, and thus increase the transfer efficiency. Also, the noxious odors can be reduced or eliminated by scrubbing the gas released from the pond, using a known supplemental process.
It has also been known, in the prior art, to capture unreacted bubbles and to recycle them. But the only known system for doing so employs a motor-driven turbine, similar to one of the devices mentioned above, which creates a swirling downdraft in the water, trapping the oxygen and driving it down. The latter system has the disadvantages that it requires electricity for use, it requires maintenance, and it is expensive to purchase and install.
The present invention provides a method and apparatus which substantially reduces the amount of wasted oxygen, and which therefore effectively enhances the efficiency of a water treatment facility.